Hydrogeochemical exploration at Tsumeb

Doctoral Thesis


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University of Cape Town

The Precambrian Otavi dolomites in the district of Tsumeb in Namibia are host to base metal mineralization but large tracts of these rocks occupy a mature pediplain and are deeply buried by transported overburden of Kalahari age. This sandy material makes exploration for minerals very difficult. Hydrogeochemistry was considered as a possible answer to the problem of prospecting in this region but a major obstacle was the lack of an undisturbed site for orientation studies. The Tsumeb mine was therefore used as a "surrogate" orientation target and water samples collected there were analyzed for about forty elements or ions. The data were plotted on long-sections through the orebody and examined with the aid of simple statistical calculations. The Na/Cl⁻ ratio proved to be a powerful index for identifying groups of waters with a common origin. It appeared that the most important of these groups of waters were not natural groundwaters but were being recycled. Millions of litres of water are pumped from the mine daily and a significant proportion finds its way back into the workings through seepage. This conclusion was supported by chemical data and by calculations in which the rates of pumping and recharge were balanced. Over the years the process of recycling has caused a rise in the concentrations of sodium, chloride and sulphate in the water arriving at the sumps in the mine. The other major components of these waters - Ca, Mg and HCO₃⁻ - are not affected to the same extent by recycling because of controls imposed by the carbonic system. It was not generally possible to establish rational links between the distributions of the trace elements, the major groups of related waters and the proximity of ore. Although some trace elements tended to be more abundant in samples from the upper parts of the mine, where exposure to broken, weathered ore is greatest, the value of this association was diminished by the fact that these waters are recycled effluents that cannot be equated with the natural groundwaters that would be in contact with any undiscovered ore-bodies similar to the Tsumeb lode. This conclusion was reinforced by the fact that some of the trace elements were relatively abundant in waters that were comparatively saline but which had not been exposed to oxidized ore. It was concluded that it would not be possible to interpret the trace element data from the mine waters unambiguously. Nevertheless it was clear that sulphate, Cu, Pb, Zn, Hg, Cd, Se, Te and perhaps U were being mobilized from the ore and that these were therefore all good "candidate'' pathfinders. Less probable candidates were Mo, Ge, As, Na and chloride. A second orientation study was then undertaken, using normal groundwaters from boreholes and springs on surface. Sampling was concentrated around the Tsumeb and Kombat mines as targets and waters were also obtained from background areas where dolomites were exposed. The chemical data were plotted on maps by computer and were examined with the aid of simple statistical calculations such as the cumulative frequency distribution. Although anthropogenic effects were apparent at both mines they were not severe and it was possible to be more confident about the prospective roles of the various chemical species in hydrogeochemical exploration for ores in the Otavi dolomites. These prospective roles may be summarized as follows: undetectable or of virtually no interest: pH, temperature, phosphate, K, Ti, Fe, li, Rb, Cs, V, Cr, Mo, Ag, Au, Cd, Hg, Tl, Ge, As, Sb, Bi, Se and Te; useful as iridicators of regional hydrogeochemical features not directly related to mineralization: Si, F⁻, Sr and Al; weak regional pathfinders: Ni and Co; probably good regional pathfinders: sulphate, Cu, Pb and Zn. Bicarbonate, Cl⁻, Na, Ca and Mg are not pathfinders but it is useful to have these data when considering the nature and significance of the samples and the abundances in them of the trace elements. Despite the identification of a suite of pathfinders it was not feasible to establish orientation criteria such as contrast ratios. An attempt was made to apply the findings of this orientation survey in an area to the northeast of Tsumeb, where dolomites are buried under thick sand and calcrete. An area of five thousand square kilometres was selected and all usable sources of groundwater within it were sampled. The following data were collected: total dissolved solids, pH, Ca, Mg, Na, bicarbonated chloride, sulphate, Cu, Pb and Zn. Hydrological data suggested that the regional movement of groundwater was from the exposed dolomites into the pediplain. The hydrogeochemical data showed that during this migration the major element composition of the groundwater changed ("metamorphosed") radically. Because of this metamorphism, very severe difficulties stood in the way of applying in the sandy pediplain what had been learned in the areas of well-exposed dolomite. An attempt was therefore made to provide tighter control between the lithology of the geological profile and the hydrogeochemistry of the associated formational waters in the pediplain. This work consisted of (a) a sampling program to try to ''fingerprint" specific formational waters. This proved to be impossible. (b) drilling four diamond drill holes near selected anomalies and studying the cores in detail. These revealed that the Kalahari beds were very thick (46 to >103m). In all cases the base of the overburden was far below the present water table and in some cases the overburden was underlain by unmineralized rocks other than dolomite. These results indicated that the hydrogeochemical anomalies were probably spurious and it was concluded that hydrogeochemical exploration in this kind of Kalahari terrane was not practicable.